1. Field of the Invention
This invention relates to photocatalytic surfaces used in the process of photoelectrolysis, photovoltaics, and photocatalysis, and more specifically to induction and management of stress in a thin titania film photocatalytic surface to match the band gap of the titania more efficiently with the solar spectrum at the earth's surface for photoelectrolysis, photovoltaics, and photocatalysis.
2. Description of the Related Art
The ills of our carbon-based energy are well-known: pollution of land and oceans, air pollution, and the global warming that is likely caused by the latter. In addition, there is the growing dependence on foreign oil (presently at 46%, up from 27% during the Oil Embargo during the Carter administration) with the economic, political, and human costs that result from that dependence. Hydrogen has been gradually emerging as the fuel of choice for the future and perhaps even the very near future. Fuel cell technology has recently advanced exponentially, with plans for miniature fuel cells to replace batteries in the ever power-hungry personal digital devices, and for combustion engines for automobiles in which hydrogen is the fuel. This last important application has made great progress in that the hydrogen can now be safely and efficiently stored in a host of metal hydride based materials, with the hydrogen being piped to or stored at local filling stations, with the associated cost and danger. In another approach, the hydrogen is split at the engine from toxic hydrogen-bearing liquids such as gasoline and alcohols.
Ultimately, for a hydrogen-based energy to be completely beneficial, one would like to be able to split our most abundant resource, water, with a renewable energy source. Many have turned to solar cells to provide the electricity for electrolysis of water as a way to provide a stable and efficient storage for solar energy, with the stored hydrogen (adsorbed in a metal hydride, Ovshinsky et al) later used to create electric power in a fuel cell. However, the losses of the solar cell in converting sunlight to electricity, combined with the losses in the electrolytic splitting of water into hydrogen and oxygen, make for low efficiency overall. Further, the cost of the apparatus and lifetime of the components make the economic viability dim at this time.
A promising path and highly sought-after goal is to use sunlight directly to split water. The free energy required for decomposing water into gaseous H2 and O2 is just 1.23 eV, so this seems possible given that the peak of the solar spectrum is about 2.4 eV (ref. Mavroides). However, the threshold energy for this reaction is 6.5 eV, so direct photodissociation is not possible. However, Honda and Fujishima (Nature 238, 37 (1972) showed that the threshold energy required can be greatly reduced by introducing a photocatalytic semiconductor surface, such as titania. Immersing single crystal titania (n-type) and Pt electrodes in an aqueous electrolyte, connected externally to form an electrolytic cell, they observed development of gaseous oxygen at the titania electrode and gaseous hydrogen at the Pt electrode when the cell was illuminated. (In other photoelectrolytic cells, hydrogen collects at the semiconductor cathode and oxygen collects at the conducting anode, with a membrane preventing their recombining.) However, while they succeeded in activating titania as a photocatalyst, they required artificial light, such as a xenon lamp, with a photon energy of greater than 3.0 eV, the energy gap of titania. Even so, their energy conversion efficiencies were low. Further, such light is in the ultraviolet part of the spectrum, and very little is present in sunlight at the surface of the earth (sunlight integrated over the 3 eV to 4 eV range is only 4 mW per square cm, compared to the 100 mW per square cm total in visible sunlight), so that titania photoelectrolysis with sunlight has less than 1% efficiency, and the photoelectrolysis quantum efficiency, independent of the solar spectrum, is only 1-2% unless a bias voltage is applied. For photoelectrolysis, as it is known, to spontaneously occur in sunlight, and with a practical efficiency, therefore requires the semiconductor to have a bandgap of about 1.7 electron volts (eV) in order to (1) have the energy required to split the water into hydrogen and oxygen gases, and (2) absorb at the peak of the solar spectrum for highest efficiency.
Following this work, others (Turner and Warren) have investigated semiconductor alloys or compounds with lower bandgaps. For example, p-type GaInP2 has a bandgap of 1.8 to 1.9 eV, which would work adequately in sunlight to produce a photocurrent that can be used to break down water into hydrogen and oxygen. However, they found that surface treatments in the form of metallated porphyrins and transition metals, such as compounds of ruthenium, were necessary to suppress the band edge migration and allow band edge overlap to occur. Without this treatment, hydrogen and oxygen cannot be produced because the conduction band and the Fermi level of the semiconductor do not overlap the redox potentials of water, i.e. when light shines on the semiconductor, electrons build up on the surface, shifting the band edges and Fermi level further away from the overlap of the water redox potentials. The long term surface stability of these surface treatments are not known.
Other attempts at photoelectrolytic cells with lower bandgap semiconductors typically (1) are corrosive in water, and (2) require a bias voltage, supplied by a conventional power source or by a photovoltaic cell or photodiode. The corrosion problem has been reduced by using platinum as the anode, and/or by combining different semiconductors. This again reduces economic viability.
The titania electrode in the Honda/Fujishima cell has the important advantage that it does not undergo anodic dissociation in water, and titania is much less expensive than other semiconductors. Mavroides, Kafalas, and Kolesar demonstrated somewhat higher efficiency titania cells using n-type SrTiO3 for its smaller electron affinity, after having confirmed the Honda/Fujishima results with titania in earlier work. They achieved 10% maximum quantum efficiency, an order of magnitude higher than for titania, but with light with energy hυ (where h is Planck's constant and υ is the light frequency) at 3.8 eV, compared to 3.0 eV required for titania. They believed this increase in efficiency was the result of band bending at the anode surface that is about 0.2 eV larger than for titania, resulting from the smaller electron affinity of SrTiO3. In their energy-level model for photoelectrolysis, the semiconductor serves as only the means for generating the necessary holes and electrons, without itself reacting chemically. In their model, the low quantum efficiency of titania is not due to inefficient carrier transfer, as others had shown that this was close to 100% with platinized —Pt cathodes and illuminated titania anodes, but rather to insufficient band-bending at the titania surface to cause efficient separation of the electron-hole pairs. The complete process, according to their model as in Ref. 2, (which is in substantial agreement with models of other researchers), is that photoelectrolysis occurs because electron-hole pairs generated at the semiconductor surface upon absorption of illumination with the required photon energy are separated by the electric field of the barrier, in the form of the energy-band bending at the surface, preventing recombination. The electrons move into the bulk of the anode and then through the external circuit to the cathode. There, they are transferred to the H2O/H2 level of the electrolyte and hydrogen gas is released:2e−+2H2O→H2+2OH−  (1)
Oxygen is produced at the same time as holes are transferred from the anode surface to the OH−/O2 level of the electrolyte, as:2p++2OH−→½O2+H2O  (2)
In other work that is farther a field from this application, Graetzel invented a titania solar photovoltaic cell in which the functions of absorption of light and the separation of the electric charges (“electrons” and “holes”) are not both performed by the semiconductor (titania in this case). Instead, the light absorption is performed by a dye monolayer that is adsorbed onto titania particles, in one case, and onto titania nano-crystals, in another case. In this way he avoids the problem of titania's 3.0 eV bandgap. This technology is now being commercialized by, for example, Sustainable Technologies International. Others have followed his lead and replaced the dye absorber with quantum dot particles attached to the titania particles, where the quantum dots perform the light absorption (QD Photovoltaics, The University of Queensland). In all of this work, however, there is no attempt to alter the bandgap of the titania. Also, the titania layer is required to be microns thick, and is applied as a sol-gel. Such a process requires solvents and temperatures incompatible with polymer substrates. Further, an electrolyte is required to fill the porous gaps in the titania matrix and complete the cell. This electrolyte is non-aqueous and somewhat volatile, so packaging, cell lifetime, and effect on the environment remain problematic. Efficiencies are reported to be around only 5% at this point. Most importantly, such a device provides no direct access to the titania photocatalytic surface, and so cannot be used for hydrogen production, detoxification, or disinfection.
Still further a-field is work by researchers at Oxford's Physics and Chemistry Departments, who are devising “inverted” photonic bandgap (PBG) crystals comprising polycrystalline titanium dioxides (titania), while earlier researchers achieved the same with self-assembled titania nano-spheres. Here, the bandgap is determined by the relative indices of refraction of the titania spheres and the empty or lower index media around and in between the spheres, the size of the spheres, and their geometrical arrangement. Again, there is no attempt to alter the bandgap of the titania spheres themselves, and the application is for directing, absorbing, and otherwise controlling light of a certain wavelength. The titania is used for its high refractive index of 2.4 to 2.6, which provides the desired index ratio of greater than 2 to if the immersion medium is air with in index of unity.
So, titania has also been shown to have use in photovoltaic devices. And in addition to photoelectrolysis for hydrogen production, titania's photocatalytic properties have been shown to have beneficial application to disinfection by killing biological organisms, and detoxification by breaking down toxic chemicals. It will be seen that the invention disclosed herein, by enabling titania to function well in visible light, such as sunlight, also applies to photovoltaics, disinfection, and detoxification.
In all of the above work, titania is either in the form of a slab cut from a crystal, and can be either of the most common polymorphs rutile or anatase, or is a thick film resulting from a sol gel process, or else are small particles of crystalline titania either in suspension or hot-pressed into a solid. No one is using, to our knowledge, titania in the form of a thin film deposited in a vacuum coating process.